High voltage alternating current (AC) power capacitors are designed to meet the mechanical, electrical, and performance requirements of high voltage high frequency AC electrical circuits. Such capacitors commonly used in electrical circuits carrying peak voltages of, for example, 1400 Vpeak and electrical current of 3000 Arms are prone to heating by three main mechanisms.
The first cause of heating of the capacitor are dielectric losses which originate from the physical and chemical properties of the dielectric material used in as an insulator in the capacitor.
The second source of heat are Ohmic loses. These losses are created by the current which flows on the different resistances (usually referred to as ESR, ESL) of the capacitor. For example, in a common high and medium frequency (e.g., 1 kHz to 1 MHz) power capacitor each 500 kVAr reactive power can generate a loss of 500 to 1000 Watt from dielectric and Ohmic losses in the form of heat.
The last source of heat, are the inductive losses. Depending on the method of mounting, the capacitor is prone to induction heating. High currents which flow from the capacitor towards the collector or consumer and/or inductor create a magnetic field which can heat the capacitor by induction.
Heating of the capacitors by the three mechanisms described above limits the number of capacitors one can use in a high voltage alternating current (AC) circuit as well as the configuration in which the capacitors can be lined up. For example, certain configurations of mounting more than one capacitor to a bus such as, for example, in parallel, may bring one or more capacitors, e.g., the first or last in the series, to overheat. Derating the capacitors and therefore using a larger number of capacitors may be needed in a system due to overheating of the capacitors.
Solutions currently practiced for cooling capacitors include running a liquid coolant through an individual capacitor or mounting capacitors on cooling busses that dissipate the heat via conduction.
Despite all of the described above measures induction heating capacitors still exhibit significant power dissipation due to the enormous AC currents they must carry and the magnetic field which is created by these currents. Therefore an important factor in their design is directing the currents in such a way that minimizes the stray effects, such as heating by induction of the capacitor itself and allowing the effective removal of heat from within the capacitor and hence extend the life of the capacitor.
The following US Patents and Patent Application Publications describe different power capacitors and methods of using such capacitors U.S. Pat. No. 1,599,853, U.S. Pat. No. 4,315,299, U.S. Pat. No. 4,317,159, U.S. Pat. No. 4,719,539, U.S. Pat. No. 5,953,201, U.S. Pat. No. 6,894,886, U.S. Pat. No. 8,416,556, US 2007/0242413, US 2009/0273916, European Patents EP 0214788, EP1263007, and Patent Cooperation Treaty Publications WO 2008/092073, WO 2009/116046 and WO 2016/000786.